Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion battery) is charging, Na+ (or Li+) ions de-intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today; however lithium is not a cheap metal to source and is considered too expensive for use in large scale applications. By contrast sodium-ion battery technology is still in its relative infancy but is seen as advantageous; sodium is much more abundant than lithium and some researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless a lot of work has yet to be done before sodium-ion batteries are a commercial reality.
NaNi0.5Mn0.5O2 is a known Na-ion material in which the nickel is present as Ni2+ while the manganese is present as Mn4+. The material is ordered with the Na and Ni atoms residing in discrete sites within the structure. The nickel ions (Ni2+) are a redox element which contributes to the reversible specific capacity and the manganese ions (Mn4+) play the role of a structure stabilizer. Compound NaNi0.5Ti0.5O2 is analogous to NaNi0.5Mn0.5O2 in that the Ni2+ ions provide the active redox centre and the Ti4+ ions are present for structure stabilization. There is plenty of literature describing the preparation of NaNi0.5Mn0.5O2 (and to a lesser extent NaNi0.5Ti0.5O2) as the precursor for making LiNi0.5Mn0.5O2 and LiNi0.5Ti0.5O2 by Na→Li ion exchange for Li-ion applications. A direct synthesis method to make these Li materials may yield undesirable disordered materials, for example, as a result of the lithium and nickel atoms sharing structural sites. However, recent electrochemical studies reported by Komaba et al Adv. Funct. Mater. 2011, 21, 3859 describe the sodium insertion performance of hard-carbon and layered NaNi0.5Mn0.5O2 electrodes in propylene carbonate electrolyte solutions. The results obtained show that NaNi0.5Mn0.5O2 exhibits some reversible charging and discharging ability, unfortunately however the capacity of the material fades by 25% or more, after only 40 cycles which makes the use of this material extremely disadvantageous for rechargeable energy storage applications.
Work is now being undertaken to find even more efficient electrochemically active materials, which have large charge capacity, are capable of good cycling performance, highly stable, and of low toxicity and high purity. Of course, to be commercially successful, the cathode materials must also be easily and affordably produced. This long list of requirements is difficult to fulfil but it is understood from the literature that the active materials which are most likely to succeed are those with small particle size and narrow size distribution, with an optimum degree of crystallinity, a high specific surface area, and with uniform morphology.
Prior art, for example PCT/GB2013/051822 and PCT/GB2013/050736, also describes that electrochemical activity is substantially improved in both specific capacity and cathode material stability, for active materials with metal constituents of certain defined oxidation states and further particularly for active materials with an O3 layered oxide crystal structure.
As described below, the present Applicant has designed a novel series of compounds which are straightforward to manufacture and easy to handle and store. Further the invention provides cost effective electrode materials, particularly cathode materials, for use in a sodium ion, a lithium-ion or a potassium-ion battery. Particularly advantageous is the high reversible discharge capacity observed even with a heavy metal addition. The cyclability of the materials of the present invention is also reasonably high.
Therefore, the first aspect of the present invention provides compounds of the formula:AUM1VM2WM3XO2±δ                wherein        A is one or more alkali metals, preferably selected from sodium, lithium and potassium;        M1 comprises one or more redox active metals with an oxidation state in the range +2 to +4, preferably with an average oxidation state of +2 or +3;        M2 comprises tin, optionally in combination with one or more transition metals, preferably with an oxidation state in the range +3 to +5 and further preferably with an average oxidation state of +4;        M3 comprises one or more transition metals either alone or in combination with one or more non-transition elements selected from alkali metals, alkaline earth metals, other metals and metalloids, with an oxidation state in the range +1 to +5, preferably with an average oxidation state of +3;        wherein        the oxidation state of M1, M2, and M3 are chosen to maintain charge neutrality;        and further wherein        δ is in the range 0≤δ≤0.4, preferably 0≤δ≤0.2;        U is in the range 0.3<U<2, preferably 0.5<U<2, further preferably 0.5<U≤1;        V is in the range 0.1≤V<0.75;        W is in the range 0<W<0.75, preferably 0.1<W<0.75; further preferably 0.125<W<0.75;        X is in the range 0≤X<0.5;        and (U+V+W+X)<4.0.        
The above formula includes compounds that are oxygen rich or oxygen deficient. Further the oxidation states may or may not be integers i.e. they may be whole numbers or fractions or a combination of whole numbers and fractions.
Preferably the sum of the average oxidation state of (A+M1+M2+M3) is equal to the oxygen charge; i.e. it achieves charge neutrality with the oxygen content.
Further preferably (U+V+W+X) is 3.5.
Yet further preferably M3 comprises one or more transition metals either alone or in combination with one or more non-transition elements selected from alkali metals, alkaline earth metals, other metals an metalloids, with an oxidation state in the range +2 to +4.
Preferably, M1 comprises one or more metals selected from nickel, manganese, cobalt, iron and chromium.
Preferably, M2 comprises tin, optionally in combination with one or more metals selected from magnesium, copper, titanium, vanadium, chromium and manganese, with a preferred oxidation state in the range +3 to +5 and further preferably with an average oxidation state of +4.
Preferably, M3 comprises one or more transition metals selected from titanium, vanadium, niobium, tantalum, hafnium, chromium, molybdenum, tungsten, manganese, iron, osmium, cobalt, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, scandium, yttrium, zirconium, technetium, rhenium, ruthenium, rhodium, iridium and mercury; and optionally one or more non-transition elements selected from: alkali metals including lithium, sodium and potassium; other metals including aluminium, gallium, indium, lead, bismuth and thallium; alkaline earth metals including magnesium, calcium, beryllium, strontium and barium; and metalloids including boron, silicon, germanium, arsenic, antimony and tellurium.
When M3 includes one or more alkali metals, the alkali metal element(s) may be the same or different from the one or more alkali metals defined in A, however, A and M3 represent two different and distinct crystallographic sites within the structure of the compounds of the invention. As such, the alkali metals in one site may not be regarded as interchangeable with the alkali metal in the other site.
In additionally preferred compounds of the present invention, M2≠M3.
Preferred compounds of the present invention are of the formula:AUM1VM2WM3XO2                 i.e. δ=0        wherein        A is one or more alkali metals, preferably selected from sodium, lithium and potassium;        M1 comprises one or more redox active metals with an oxidation state in the range +2 to +4, preferably with an average oxidation state of +2 or +3;        M2 comprises tin, optionally in combination with one or more transition metals; with a preferred oxidation state in the range +3 to +5 and further preferably with an average oxidation state of +4;        M3 comprises one or more transition metals either alone or in combination with one or more non-transition elements selected from alkali metals, alkaline earth metals, other metals and metalloids, with an oxidation state in the range +1 to +5, and preferably with an average oxidation state of +3;        wherein        the oxidation state of M1, M2, and M3 are chosen to maintain charge neutrality;        and further wherein        U is in the range 0.3<U<2, preferably 0.5<U<2, further preferably 0.5<U≤1;        V is in the range 0.1≤V<0.75;        W is in the range 0<W<0.75, preferably 0.1<W<0.75; further preferably 0.125<W<0.75;        X is in the range 0≤X<0.5;        and (U+V+W+X)<4.0.        
Preferred compounds of the present invention include:
e.g.NaNi1/2Mn1/4Sn1/8Ti1/8O2 
Especially preferred compounds of the present invention include:    NaNi1/2Mn1/4Sn1/4O2     NaNi1/3Mn1/3Sn1/6Mg1/6O2     NaNi1/2Mn1/4Sn1/8Ti1/8O2     NaNi1/4Mn4/12Sn3/12Na1/6O2     NaNi1/3Mn1/6Sn1/6Mg1/6Ti1/6O2     NaNi1/2Mn1/4Sn1/8Ti1/8O2     NaNi1/2Ti1/4Sn1/4O2     Na0.95Ni0.3167Mn0.3167Mg0.1625Sn0.2018O2     Na0.95Ni0.3167Sn0.3167Mg0.1625Ti0.2018O2     NaNi1/2Sn1/2O2     NaNi1/2Ti1/4Sn1/4O2     NaNi1/2Ti3/8Sn1/8O2     NaCo1/8Ni1/8Na1/6Mn4/12Sn3/12O2     NaCo1/2Mn4/12Sn3/12Na1/6O2     NaCo1/2Mn1/4Sn1/8Ti1/8O2     NaCo1/2Sn1/2O2     NaCo1/2Sn1/4Ti1/4O2     NaMn1/2Sn1/2O2     NaMn1/2Ti1/4Sn1/4O2     NaFe1/2Mn1/4Sn1/8Ti1/8O2     NaFe1/2Sn1/2O2     NaFe1/4Mn4/12Sn3/12Na1/6O2     NaNi1/2Mn1/4Sn1/8Ti1/8O1.9     NaNi1/2Ti3/8Sn1/8O1.95     Na9/10Li1/10Ni1/2Mn1/4Sn1/8Ti1/8O2     Na8/10Li2/10Ni1/2Mn1/4Sn1/8Ti1/8O2     Na7/10Li3/10Ni1/2Mn1/4Sn1/8Ti1/8O2     Na5/10Li5/10Ni1/2Mn1/4Sn1/8Ti1/8O2     NaNi1/4Na1/6Mn13/24Sn1/24O2     NaNi1/4Na1/6Mn1/12Ti5/12Sn1/12O2     NaNi1/4Na1/6Mn2/12Ti4/12Sn1/12O2.
Further, extremely preferred compounds of the present invention include:    NaNi1/2Mn1/4Sn1/4O2     NaNi1/3Mn1/3Sn1/6Mg1/6O2     NaCo1/8Ni1/8Na1/6Mn4/12Sn3/12O2     NaNi1/3Mn1/6Sn1/6Mg1/6Ti1/6O2     NaNi1/2Mn1/4Sn1/8Ti1/8O2     NaNi1/2Ti1/4Sn1/4O2     Na0.95Ni0.3167Mn0.3167Mg0.1625Sn0.2018O2     Na0.95Ni0.3167Sn0.3167Mg0.1625Ti0.2018O2     NaNi1/2Sn1/2O2     NaNi1/2Ti3/8Sn1/8O2     NaNi1/2Mn1/4Sn1/8Ti1/8O1.9     NaNi1/2Ti3/8Sn1/8O1.95     NaNi1/4Na1/6Mn13/24Sn1/24O2     NaNi1/4Na1/6Mn1/12Ti5/12Sn1/12O2     NaNi1/4Na1/6Mn2/12Ti4/12Sn1/12O2.
Alternative preferred compounds of the present invention are of the formula:AUM1VM2WM3XO2±δ                wherein 0<δ≤0.4; preferably 0<δ≤0.2; oxygen deficient compounds e.g. O1.9 are particularly preferred        wherein        A is one or more alkali metals, preferably selected from sodium, lithium and potassium;        M1 comprises one or more redox active metals with an oxidation state in the range +2 to +4, preferably with an average oxidation state of +2 or +3;        M2 comprises tin, optionally together with one or more transition metals, preferably with an oxidation state in the range +3 to +5 and further preferably with an average oxidation state of +4;        M3 comprises one or more transition metals either alone or in combination with one or more non-transition elements selected from alkali metals, alkaline earth metals, other metals, and metalloids, with an oxidation state in the range +1 to +5, and preferably with an average oxidation state of +3;        wherein        the oxidation state of M1, M2, and M3 are chosen to maintain charge neutrality;        and further wherein        U is in the range 0.3<U<2, preferably 0.5<U<2, further preferably 0.5<U≤1;        V is in the range 0.1≤V<0.75;        W is in the range 0<W<0.75, preferably 0.1<W<0.75; further preferably 0.125<W<0.75;        X is in the range 0≤X<0.5;        and (U+V+W+X)<4.0.        
Alternative preferred compounds of the present invention include:NaNi1/2Mn1/4Sn1/8Ti1/8O1.9 NaNi1/2Ti3/8Sn1/8O1.95 
In a second aspect, the present invention provides an electrode comprising an active compound of the formula:AUM1VM2WM3XO2±δ                wherein        A is one or more alkali metals, preferably selected from sodium, lithium and potassium;        M1 comprises one or more redox active metals with an oxidation state in the range +2 to +4, preferably with an average oxidation state of +2 or +3;        M2 comprises tin, optionally in combination with one or more transition metals, preferably with an oxidation state in the range +3 to +5 and further preferably with an average oxidation state of +4;        M3 comprises one or more transition metals either alone or in combination with one or more non-transition elements selected from alkali metals, alkaline earth metals, other metals and metalloids, with an oxidation state in the range +1 to +5, preferably with an average oxidation state of +3;        wherein        the oxidation state of M1, M2, and M3 are chosen to maintain charge neutrality;        and further wherein        δ is in the range 0≤δ≤0.4, preferably 0≤δ≤0.2;        U is in the range 0.3<U<2, preferably 0.5<U<2, further preferably 0.5<U≤1;        V is in the range 0.1≤V<0.75;        W is in the range 0<W<0.75, preferably 0.1<W<0.75; further preferably 0.125<W<0.75;        X is in the range 0≤X<0.5;        and (U+V+W+X)<4.0.        
The elements M1, M2 and M3, their average oxidation state and the values of U, V, W and X, are the same as described above in relation to the compounds of the present invention. The Applicants have found that in the voltage ranges they have investigated, tin per se is mostly electrochemically inactive. Nevertheless, the Applicants have found that the presence of tin has no adverse effects of the specific energy density of the active compounds of the present invention, and moreover the addition of tin is found to have a stabilising effect on the structure of these active materials when used in the electrodes of the present invention. Indeed, the presence of tin is found to be highly advantageous because it promotes higher than expected specific capacities than have been any observed previously for other layered oxide materials. Such a result is extremely surprising, especially in respect of the sodium/tin-containing compounds of the present invention; in the case of such sodium/tin-containing compounds one would expect that the high atomic weight of sodium would reduce the specific capacity but as demonstrated below this is not what is observed in practice. The presence of tin is further advantageous because it improves the electrochemical stability on cycling and produces active materials which are capable of being charged and recharged with only a moderate reduction in cycling capacity. Moreover, all of these advantages are obtained when only fractional amounts of tin are present in the active compounds.
Preferred electrodes of the present invention comprise active compounds selected from one or more of: e.g.NaNi1/2Mn1/4Sn1/8Ti1/8O2 
Especially preferred electrodes of the present invention comprise active compounds selected from one or more of:    NaNi1/2Mn1/4Sn1/4O2     NaNi1/3Mn1/3Sn1/6Mg1/6O2     NaNi1/2Mn1/4Sn1/4O2     NaNi1/3Mn1/3Sn1/6Mg1/6O2     NaNi1/2Mn1/4Sn1/8Ti1/8O2     NaNi1/4Mn4/12Sn3/12Na1/6O2     NaNi1/3Mn1/6Sn1/6Mg1/6Ti1/6O2     NaNi1/2Mn1/4Sn1/8Ti1/8O2     NaNi1/2Ti1/4Sn1/4O2     Na0.95Ni0.3167Mn0.3167Mg0.1625Sn0.2018O2     Na0.95Ni0.3167Sn0.3167Mg0.1625Ti0.2018O2     NaNi1/2Sn1/2O2     NaNi1/2Ti1/4Sn1/4O2     NaNi1/2Ti3/8Sn1/8O2     NaCo1/8Ni1/8Na1/6Mn4/12Sn3/12O2     NaCo1/2Mn4/12Sn3/12Na1/6O2     NaCo1/2Mn1/4Sn1/8Ti1/8O2     NaCo1/2Sn1/2O2     NaCo1/2Sn1/4Ti1/4O2     NaMn1/2Sn1/2O2     NaMn1/2Ti1/4Sn1/4O2     NaFe1/2Mn1/4Sn1/8Ti1/8O2     NaFe1/2Sn1/2O2     NaFe1/4Mn4/12Sn3/12Na1/6O2     NaNi1/2Mn1/4Sn1/8Ti1/8O1.9     NaNi1/2Ti3/8Sn1/8O1.95     Na9/10Li1/10Ni1/2Mn1/4Sn1/8Ti1/8O2     Na8/10Li2/10Ni1/2Mn1/4Sn1/8Ti1/8O2     Na7/10Li3/10Ni1/2Mn1/4Sn1/8Ti1/8O2     Na5/10Li5/10Ni1/2Mn1/4Sn1/8Ti1/8O2     NaNi1/4Na1/6Mn13/24Sn1/24O2     NaNi1/4Na1/6Mn1/12Ti5/12Sn1/12O2     NaNi1/4Na1/6Mn2/12Ti4/12Sn1/12O2.
Further, extremely preferred electrodes of the present invention comprise active compounds selected from one or more of:    NaNi1/2Mn1/4Sn1/4O2     NaNi1/3Mn1/3Sn1/6Mg1/6O2     NaCo1/8Ni1/8Na1/6Mn4/12Sn3/12O2     NaNi1/3Mn1/6Sn1/6Mg1/6Ti1/6O2     NaNi1/2Mn1/4Sn1/8Ti1/8O2     NaNi1/2Ti1/4Sn1/4O2     Na0.95Ni0.3167Mn0.3167Mg0.1625Sn0.2018O2     Na0.95Ni0.3167Sn0.3167Mg0.1625Ti0.2018O2     NaNi1/2Sn1/2O2     NaNi1/2Ti3/8Sn1/8O2     NaNi1/2Mn1/4Sn1/8Ti1/8O1.9     NaNi1/2Ti3/8Sn1/8O1.95     NaNi1/4Na1/6Mn13/24Sn1/24O2     NaNi1/4Na1/6Mn1/12Ti5/12Sn1/12O2     NaNi1/4Na1/6Mn2/12Ti4/12Sn1/12O2.
Advantageously, the electrodes according to the invention are used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s) or mixtures thereof.
As well as in electrodes, the active materials according to the present invention are suitable for use in many different applications, for example in energy storage devices such as rechargeable batteries, electrochemical devices and electrochromic devices. The active materials according to the present invention may be used as part of an electrode in these energy storage devices, but this need not necessarily be the case.
In a third aspect, the present invention provides an energy storage device that utilises one or more active materials according to the present invention as described above, and particularly an energy storage device for use as one or more of the following: a sodium and/or lithium and/or potassium ion cell; a sodium and/or lithium and/or potassium metal cell; a non-aqueous electrolyte sodium and/or potassium ion; an aqueous electrolyte sodium and/or lithium and/or potassium ion cell.
The novel compounds of the present invention may be prepared using any known and/or convenient method. For example, the precursor materials may be heated in a furnace so as to facilitate a solid state reaction process.
A fourth aspect of the present invention provides a particularly advantageous method for the preparation of the compounds described above comprising the steps of:    a) mixing the starting materials together, preferably intimately mixing the starting materials together and further preferably pressing the mixed starting materials into a pellet;    b) heating the mixed starting materials for example in a furnace, at a preferred temperature of between 400° C. and 1500° C., further preferably at a temperature of between 500° C. and 1200° C., for between 0.5 and 20 hours; and    c) allowing the reaction product to cool.
Preferably the reaction is conducted under an atmosphere comprising one or more selected from ambient air and any other gaseous medium. Examples of a suitable gaseous medium include one or more selected from an inert gas, nitrogen and oxygen. Where two or more gases are used, they may be combined to produce a mixture, or alternatively, the two or more gases may be used sequentially either singly or in any combination, and in any order. Preferably an ambient air atmosphere is followed by an atmosphere of nitrogen.
In a preferred method of the present invention, steps a), and b) of the reaction may be performed under an atmosphere of ambient air or under a partial oxygen-containing atmosphere and then cooling in step c) may be performed under an ambient air or partial oxygen-containing atmosphere, or in a non-oxidising atmosphere e.g. under nitrogen gas or quenched in liquid nitrogen. The non-stoichiometric levels of oxygen in the product (i.e. compounds in which δ≠0) can be tailored by the choice of cooling step used in the method of the present invention; that is, different cooling rates at different temperatures in different atmospheric conditions gives a different amount of oxygen in the final product.
A particularly convenient method to achieve oxygen deficient compounds of the present invention is to perform step c) of the above method under nitrogen as this results in the oxygen deficiency being fixed.